Field of the Invention
The present invention relates to a vibration actuator reduced in cost and size, and an electronic device equipped with the vibration actuator.
Description of the Related Art
A vibration actuator, in general, brings a vibration element and a driven element into pressure contact with each other, and causes a frictional driving force to act between the vibration element and the driven element by vibration excited in the vibration element, to thereby move the vibration element and the driven element relative to each other. As an example of the vibration actuator, there is known one having a structure that brings tops of two protrusions provided on a vibration element and a driven element into pressure contact with each other (see e.g. Japanese Patent Laid-Open Publication No. 2015-104144). The vibration element as a component of the vibration actuator described in Japanese Patent Laid-Open Publication No. 2015-104144 includes a plate-shaped elastic body, the two protrusions provided on one of surfaces of the elastic body, and a piezoelectric element (electromechanical energy conversion element) provided on (joined to) the other of the surfaces of the elastic body opposite to the surface provided with the protrusions.
When driving the vibration actuator having this structure, two AC voltages having a phase difference therebetween are applied from a drive circuit to the piezoelectric element as driving voltages. By applying the driving voltages, vibrations in two bending vibration modes are simultaneously excited in the vibration element, and extremity ends of the two protrusions are caused to perform elliptic motion within a plane including a direction connecting between the two protrusions and a direction in which the two protrusions protrude from the elastic body, whereby the driven element receives a frictional driving force from the two protrusions. Thus, for example, in the vibration actuator having the vibration element fixed to a fixing member, it is possible to move the driven element in the direction connecting the two protrusions.
The vibration actuator described in Japanese Patent Laid-Open Publication No. 2015-104144 has a pressure plate disposed on a bottom surface (surface opposite to the surface joined to the elastic body) of the piezoelectric element, with a felt therebetween, and employs pressure means for pressing the pressure plate toward the driven element using a pressure spring, to thereby press the vibration element against the driven element. In doing this, a guide portion is provided on the fixing member that holds the driven element, for receiving a reaction force of a pressure force, whereby it is possible to realize a stable pressurized state of the vibration element without hindering vibration excited in the vibration element. On the other hand, Japanese Patent Laid-Open Publication No. 2009-11058 proposes a vibration actuator that is configured, for the purpose of reduction of the size and thickness thereof, such that a magnetic force of a permanent magnet as a different pressure means generates a pressure contact force for bringing a vibration element and a driven element into pressure contact with each other.
However, the pressure means described in Japanese Patent Laid-Open Publication No. 2015-104144 has a problem that since the pressure spring and the felt are required, the number of components is increased. Further, even through the size of the vibration element is reduced, it is difficult to realize size reduction of the whole vibration actuator due to an increase in the number of components. Further, the guide portion for receiving the reaction force of the pressure force requires a structure which is large in rated load, and hence it is difficult to reduce the size of the guide portion.
On the other hand, in the pressure means described in Japanese Patent Laid-Open Publication No. 2009-11058, although the pressure spring and the component for receiving the reaction force of the pressure spring are not required, the permanent magnet for generating the magnetic force is required, and hence the manufacturing cost of the vibration actuator is increased. Further, in a case where the permanent magnet is used, sensors and other like components disposed within an apparatus for which the vibration actuator is provided may undergo malfunction due to magnetism of the permanent magnet. Therefore, this imposes a restriction that the vibration actuator should be disposed sufficiently away from the sensors and other like components, which hinders the size reduction of the apparatus. Further, differently from a DC motor and a stepping motor, the vibration actuator is non-magnetic, and hence it is possible to use the vibration actuator under an environment with a strong magnetic field, but with the use of the permanent magnet therein, there is a fear that the vibration actuator can no longer be used under the environment with a strong magnetic field.